1. Field OF The Invention
The present invention relates to a method of making polyethylene resinous open cell cellular bodies, in particular a polyethylene resin open cell cellular body that is excellent in both recyclability and heat resistance and further has a maximum degree of cell interconnection.
2. Description of the Prior Art
There has hitherto been proposed in the art, as disclosed in patent literatures JP SHO 59-23545 A and JP SHO 56-146732 A, a method of making a polyethylene resin open cell cellular body by partially decomposing a foaming (expanding) and a crosslinking agent in a foamable and crosslinkable composition of polyethylene resin material in a closed mold, then decomposing the remaining parts of the foaming and crosslinking agents under an atmospheric pressure to obtain a body with closed cells from the composition, and finally compressing the thus obtained body to cause the closed cells to be destructed.
In this conventional method, while the composition is in the closed mold in which the foaming and crosslinking agents are heated under pressure to be partially decomposed, crosslinking reaction does occur but foaming or cell formation does not take place. The foaming takes place after pressure removal. In the method, therefore, in which crosslinking completely precedes cell formation, i.e., chemical links have already been established in the process step of forming closed cells, the cell membrane becomes strong, tough and tenacious. As a result, the subsequent compressing step is difficult to interconnect isolated cells to be unable to yield a cellular body that has a degree of cell interconnection of 100% or even approaching 100%.
We have proposed several techniques designed to solve these problems met by the prior method mentioned above. These include a method as disclosed in JP SHO 62-19294 B and JP HEI 1-44499 B in which a foamable and crosslinkable composition having its base composed of ethylene acetate copolymer or low density polyethylene is heated and preformed into a preselected shape and then under atmospheric pressure is heated to cause a foaming and a crosslinking agent in the composition to be decomposed simultaneously to form a cellular body having cells therein and the cellular body is then mechanically deformed to cause the cells to be interconnected therein. In another method that we have proposed, a polyethylene resin open cell cellular material as disclosed in JP HEI 2-22345 A is further irradiated with a beam of electrons, thereby imparting heat resistance thereto. In a further method as we have disclosed in JP HEI 11-315161, a foamable and crosslinkable composition having its base composed of polyethylene derived from polymerization of ethylene catalyzed by a metallocene is heated and preformed and then under atmospheric pressure is heated to cause to a foaming and a crosslinking agent in the composition to be decomposed simultaneously to form a cellular body having cells therein, the cellular body being then mechanically deformed to cause the cells to be interconnected therein.
These methods permit making an open cell cellular body as desired that has a degree of cell interconnection as high as or nearly 100%. Despite its satisfactory degree of cell interconnection, however, a body made by any of the methods mentioned has been found still unsatisfactory. First, its crosslinked structure does not allow remelting a body on recovery after use and thus makes it unsuitable for recycling. Second, use of an ethylene acetate copolymer leaves an open cell cellular body still relatively poor in heat resistance by having a change in dimensions according to JIS K 6767 as much as xe2x88x9210 to xe2x88x9220% at 70xc2x0 C.
Proposals have also been made to make non-crosslinkable, polyethylene resin open cell cellular bodies. For example, JP SHO 60-55290 B discloses an exclusion method in which a low-density and a high-density polyethylene are mixed together on specifying their melt indices and mixing ratio, and the mixture is mixed with a volatile organic liquid for exclusion on specifying a particular extrusion temperature. JP HEI 3-8379 B discloses a method in which an olefin resin is mixed with an ionic copolymer and a foaming agent to form a resinous composition, and then the composition is excluded and foamed. In a method disclosed in JP P 10-279724, a polyolefin resin is mixed with a resinous composition of an ethylene-methacrylic acid copolymer (ionomer resin), a nucleation agent and an anti-shrink agent, which upon softening or melting by heating is supplied and mixed with a volatile foaming agent, to form a composition, and then the composition is excluded into a low pressure region.
Open cell cellular bodies made by these latter methods are indeed good in recyclability, but are poor in the degree of cell interconnection and therefore are unsuitable for use as carriers in an organism filtration tank included in an organic filterable water-purifier tank system and as a filter, which are important applications of such bodies.
It is accordingly an object of the present invention to solve the above mentioned problems met by the prior art and to provide a method of making a polyethylene resin open cell cellular body that is excellent in both recyclability and heat resistance or resistance to heat distortion and further has a maximum degree of cell interconnection.
In order to achieve the object mentioned above there is provided in accordance with the present invention in a first aspect thereof a method of making a polyethylene resin open cell cellular body, characterized in that it comprises the steps of: preparing a polyethylene resin material that is derived from polymerization of ethylene catalyzed by a metallocene compound; adding 4,4xe2x80x2-oxy bis (benzene sulfonyl hydrazide) to the polyethylene resin material to form a foamable composition and shaping the foamable composition of polyethylene resin material; heating the shaped foamable composition of polyethylene resin material to be foamed under an atmospheric pressure to form a cellular body thereof with cells therein; and mechanically deforming the said cellular body of polyethylene resin material to cause the cells to be interconnected therein, thereby producing the open cell cellular body of polyethylene resin material.
The present invention also provides in a second aspect thereof a method of making a polyethylene resin open cell cellular body, characterized in that it comprises the steps of: preparing a polyethylene resin material that is an ethylene-vinyl acetate copolymer; adding 4,4xe2x80x2-oxy bis (benzene sulfonyl hydrazide) to the polyethylene resin material to form a foamable composition and shaping the foamable composition of polyethylene resin material; heating the shaped foamable composition of polyethylene resin material to be foamed under an atmospheric pressure to form a cellular body thereof with cells therein; and mechanically deforming the said cellular body of polyethylene resin material to cause the cells to be interconnected therein, thereby producing the open cell cellular body of polyethylene resin material. In a preferred form of embodiment of the present invention in this aspect, the ethylene-vinyl acetate copolymer contains not less than 5% of vinyl acetate. It has been found that if the ethylene-vinyl acetate copolymer contains less than 5% of vinyl acetate, the foam breakability of the cellular body is significantly lowered, presumably by reason of stiffness properties of the resin.
In a specific form of embodiment of the present invention in either the first or second aspect thereof mentioned above, it is preferable that the fomable composition be freely foamed in three-dimensional directions under the atmospheric pressure. Otherwise, it has been found that there develops in the mold a pressure that acts to compress the composition, thus to hinder its foaming and to cause it to shrink.
The present invention also provides in a third aspect thereof a method of making a polyethylene resin open cell cellular body, characterized in that it comprises the steps of: adding 4,4xe2x80x2-oxy bis (benzene sulfonyl hydrazide) to a polyethylene resin material to form a foamable composition and shaping the foamable composition of polyethylene resin material; heating the shaped foamable composition of polyethylene resin material to be foamed under an atmospheric pressure to form a cellular body thereof with cells therein; mechanically deforming the said cellular body of polyethylene resin material to cause the cells to be interconnected therein, thereby producing the open cell cellular body of polyethylene resin material; and irradiating the produced open cell cellular body of polyethylene resin material with a beam of electrons, thereby rendering the same heat-resistant or resistant to heat distortion.
The present invention also provides in a fourth aspect thereof a method of making a polyethylene resin open cell cellular body, characterized in that it comprises the steps of: adding 4,4xe2x80x2-oxy bis (benzene sulfonyl hydrazide) to a polyethylene resin material to form a foamable composition and shaping the foamable composition of polyethylene resin material; heating the shaped foamable composition of polyethylene resin material to be foamed under an atmospheric pressure to form a cellular body thereof with cells therein; and mechanically deforming the cellular body of polyethylene resin material to cause the cells to be interconnected therein, thereby producing the open cell cellular body of polyethylene resin material, wherein the produced open cell cellular body of polyethylene resin material is rendered heat-resistant or resistant to heat distortion by irradiating the formed cellular body prior to mechanical deformation, with a beam of electrons.
The present invention is based on the recognition that a foaming agent 4,4xe2x80x2-oxy bis (benzene sulfonyl hydrazide) is low in decomposition temperature and high in rate of decomposition. It has now been found that this particular foaming agent if applied to the purposes of the present invention is highly suitable for forming a polyethylene resin open cell cellular body of the desired properties. The foaming agent has been found to leave its decomposed residue that has enough power advantageously to hold foam forming gases and to permit foaming to an increased extent even in a non-crosslinked structure. The decomposed residue has further been found to be effective to promote increasing the heat resistance or resistance to heat distortion of the cellular body produced.
For the polyethylene resin material in the present invention, use may be made suitably of a polyethylene resin that is derived from polymerization of ethylene catalyzed by a metallocene compound, an ethylene-vinyl acetate copolymer, a high pressure processed low density polyethylene, and so forth. A polyethylene resin that is derived from polymerization catalyzed by a metallocene compound is made up of a copolymer of ethylene and one or more xcex1-olefins of C3 to C18, which may, for example, be propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, and 1-octene. Among the various polyethylene resins, a polyethylene resin that is derived from polymerization of ethylene catalyzed by a metallocene compound, and an ethylene-vinyl acetate copolymer are particularly preferable in respect of the flexibility of an open cell cellular body.
A metallocene catalyst in general is a compound having a structure in which a transition metal is sandwiched by unsaturated compounds of xcfx80 electron system. In the present invention, there are mentioned compounds having tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium and platinum together with one or more cyclopentadienyl rings or their family members as ligands.
For ligands other than cyclopentadienyl rings, mention may be made of, for example, a cyclopentadienyl oligomer ring, indenyl ring, or cyclopentadienyl or indenyl ring having substitution with a hydrocarbon radical, substitution hydrocarbon radical or hydrocarbon substituted metalloid radical. Other than these ligands, for example monovalent chlorine or bromine anion, bivalent anion chelate, hydrocarbon radical, alkoxide, aryl-alkoxide, aryl-oxide, amide, aryl-amide, phosphide or aryl-phosphide may be coordinated and bonded to a transition metal.
These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred forms of embodiment.
The present invention will hereinafter be described in conjunction with suitable forms of embodiment thereof for methods of making an open cell cellular body.
First Form of Implementation of the Invention
100 parts by weight of a polyethylene resin that is derived from polymerization of ethylene catalyzed by a metallocene compound and/or an ethylene-vinyl acetate copolymer have added thereto preferably 1 to 30 parts by weight of a foaming agent 4,4xe2x80x2-oxy bis (benzene sulfonyl hydrazide) and also where necessary a bulking agent and a pigment. The resultant composition is kneaded by a heated mixing roll, pressure kneader, extruder or the like.
In the present invention, in order to improve the physical properties of a composition used, to function as a nucleator (to uniform and to make smaller gas bubbles), to improve defoaming and to reduce the cost, one or more additives (bulking agent or filler) may if necessary be added that do not much adversely affect the decomposition reactions of the foaming agent and that do not crosslink the resin, for example, a metal oxide such as zinc oxide, titanium oxide, calcium oxide, magnesium oxide or silicon oxide, a carbonate such as magnesium carbonate or calcium carbonate, or a fibrous material such as pulp, any of various dyes, pigments, fluorescent materials and rubber compounding ingredients of common use and so forth.
A foamable composition that is obtained in a manner as described above is loaded into a mold and under a pressure applied by a press is heated to a temperature in excess of a melting point of the resin and thereby shaped. This heat-shaping process is a process step by which the composition acquires a given shape and in which the foaming agent 4,4xe2x80x2-oxy bis (benzene sulfonyl hydrazide) does not materially decompose. Although a very small amount of the foaming agent initially decomposes to an extent that the formed body may expand twice in volume when removed from the mold, this is far from the concept of foaming and has no objection to the present invention.
Subsequent heating of the foamable composition so formed as described above under an atmospheric pressure decomposes the foaming agent 4,4xe2x80x2-oxy bis (benzene sulfonyl hydrazide). This heating under the atmospheric pressure may be carried out in a hot-air temperature controlled bath, by heating with a heater attached fast to the outer surface of a metal plate, by jacket heating with a vapor or heated oil, or in an oil, metal or salt bath.
In the present invention, atmospheric foaming as described is preferably effected freely in three-dimensional directions. It is also preferable that the lower surface of the foamable composition be laid on an anti-friction material or lubricant such as talc so that resistance to foaming may not develop. The lower surface may have applied thereto a metal plate of iron or aluminum, a metal net, a fiber net or any of various sheets surface-processed with polytetra-fluoroethylene.
The temperature for heating is set in a range, preferably of 140 to 180xc2x0 C., more preferably of 145 to 175xc2x0 C., depending on the type of a resin used. The time period for heating is preferably from 5 to 40 minutes, more preferably from 10 to 35 minutes. A decomposed residue of 4,4xe2x80x2-oxy bis (benzene sulfonyl hydrazide) tends to blacken at high temperature for long time period. Thus, heating the composition at a higher temperature and/or for a longer time period than those mentioned above blackens a resultant cellular body.
In this manner, a cellular body is obtained that has a cell membrane that can readily be ruptured if the cellular body is mechanically deformed.
A thus obtained cellular body with closed cells is next compressed and deformed using, for example, even speed rolls whereby the cell membranes are broken and an open cell cellular body is obtained with the interconnection of cells. The even speed rolls may have on their surfaces a quite large number of needles, or may be provided in their front and/or back with a roll or rolls having a very large number of needles on their surface so that these needles puncture a countless number of holes in the surface of the foam therewith, thereby facilitating interconnection of the cells.
Measuring thus obtained open cell cellular bodies with an air comparison type density meter model 1000 of Tokyo Science Inc. complying with ASTM D 2856 indicates a degree of cell interconnection of 100% or nearly 100%.
Examples of First Form of Implementation
While the present invention in this form of implementation is hereinafter described in further detail specifically with respect to certain examples thereof summarized in Table 1 given and comparative examples summarized in Table 2 given below Table 1, it should be noted that these Examples are intended in no way to limit the present invention.